Porous silicon and silicon nanocrystals (or quantum dots) have received considerable interest in the art. These materials exhibit unique physical properties, including the efficient, size-tunable emission of visible light that may be harnessed in a variety of applications. Such applications include their use in Si-based light-emitting diodes (LEDs), in the optical integration of integrated circuits, and as biological imaging contrast agents. Si nanocrystals also exhibit increased photoconductivity when illuminated, and thus may be integrated into the next generation of low cost solar cells.
Nanorods may be distinguished from other nanocrystals by their aspect ratios (the ratio of their length to their width). Thus, by definition, nanorods have aspect ratios that range from greater than 1 up to about 100. By contrast, nanowires have aspect ratios of 100 or greater, and thus may be “infinitely” long. Semiconductor nanorods are anisotropic nanocrystals which may have faceted surfaces, and which may be cylindrical or ellipsoidal in shape.
When the dimensions of semiconductor nanocrystals are of the order of the Bohr exciton diameter or the de Broglie wavelength of an electron or hole, many of the optical, electronic and physical properties of the nanocrystals become size-dependent. Consequently, semiconductor nanorods may exhibit electronic and optical properties that depend on both their thickness and length. By contrast, the corresponding optical and electronic properties of spherical nanocrystals and nanowires are typically determined by a single dimension (their diameter).
Many of the electronic and optical properties of nanorods lie somewhere in between those of spherical nanocrystals and nanowires. Thus, for example, FIG. 15 depicts the slope relationships for the size dependence (where d is thickness or diameter) of the effective band gaps (ΔEGS) in quantum wells QWs, QRs, and QDs composed of the same semiconductor material, as predicted by simple EMA-PIB models. The slope ratios are determined to be Awell/Awire/Adot=1:00:2.34:4.00. As seen therein, the nanorod zone is bounded by the zones for spherical nanocrystals and nanowires.
Semiconductor nanorods may also be distinguished from spherical nanocrystals and from nanowires by other properties, including their polarization of light. Such polarization may depend on both the length and width of the nanorods. Nanorods may also emit highly polarized light and may exhibit large permanent dipole moments. Nanorods may also exhibit other properties that differ fundamentally from spherical nanocrystals, such as their excitonic fine structure and excited state lifetimes. Nanorods may also be more suitable than other types of nanocrystals for certain applications, such as their use in lasers, which require optical gain and spontaneous emission.
Relative to nanowires, nanorods may also be more processable. By way of example, nanorods may be printed with inkjet devices, whereas nanowires are typically too long to fit through the orifices of inkjet printers. Nanorods may be produced as colloidal dispersions or mixed with polymers to form compositions which exhibit good flow properties, whereas nanowires become tangled and do not flow easily. Like spherical nanocrystals, nanorods may be dispersed easily, but exhibit unique optical and electronic properties compared to spherical nanocrystals. As with liquid crystals, nanorods may also be oriented in concentrated dispersions, with their long axes preferably aligned. This kind of orientation may be utilized in the production of optical polarization filters or fluorescent films which exhibit polarized light emission.
Monodisperse nanocrystals of many different kinds of semiconductor materials may be effectively obtained, in significant quantities, through solution-based chemical synthesis. By way of example, vapor-liquid-solid (VLS), solution-liquid-solid (SLS), and supercritical fluid-liquid-solid (SFLS) processes have been developed in the art which may be used to produce crystalline nanowires having very high aspect ratios and few crystallographic defects. Notably, all of these processes rely on the use of metal seed particles to induce the growth of semiconductor nanowires. In some cases, the synthesis allows the shape of the nanocrystals to be somewhat tunable, thus providing some control over the properties of the resulting material.
The solution-based synthesis of silicon nanocrystals, however, is very challenging, due to the relatively high crystallization energy barrier of silicon and its complicated reaction and surface chemistry. Existing methods for making silicon nanocrystals provide limited control over the dimensions of the resulting nanocrystals. Indeed, to date, colloidal Si nanorods have never been produced.
There are many examples in the literature of Si nanocrystal synthesis in solution by arrested precipitation. However, most of these methods are challenged by low yields, as the crystallization temperature for Si is relatively high and the reactions are limited by the boiling temperatures of the solvents. Solution-dispersible, quantum-size Si nanocrystals approximately 5 nm or less may also be obtained by “two-step” synthetic routes, such as plasma-assisted growth, gas or laser pyrolysis, liberation of nanocrystals by etching silicon-embedded oxides, and ultrasonication of porous Si, followed by nanocrystal capture (and often passivation) in a solvent. These methods provide effective routes to solution-dispersible Si quantum dots, but do not provide obvious methods for controlling the Si nanocrystal shape.
In contrast to these two-step methods, direct synthesis in solution has proven to be an effective method for obtaining significant quantities of nanorods and nanowires of many different semiconductors via processes such as ligand-assisted growth, oriented attachment, and nanocrystal-seeded growth. Si nanowires have been synthesized in solution, in high boiling solvents by solution-liquid-solid (SLS) growth and in high pressure, hot supercritical fluids by supercritical fluid-liquid-solid (SFLS) growth. Both of these processes rely on the use of metal nanocrystals as seeds to promote nanowire growth. Si nanowires grown by vapor-liquid-solid (VLS) methods have been integrated into single wire FETs, and the optical and electronic properties of quantum size Si nanowires have been characterized. As indicated above, however, to date, Si nanorods have not been made by a solution-based synthetic process.